My first attempt was with larger image file sizes--roughly an 8mb EPUB. That wound up becoming a 16mb MOBI.

I went back and knocked down the gifs to 3-color. This decreased my EPUB to 4mb. Calibre conversion, however, resulted in a similar 15mb MOBI size.

I unpacked the MOBI and determined that Calibre is converting my 3-color 10k gifs to 50k jpegs.

Is there anything I can do to prevent this? I'm annoyed at having to do all this tweaking to decrease Amazon's stupid "delivery fee." Then I see my file sizes increasing 2x to 4x just on conversion.

I've also tried Kindlegen and that "only" results in a 2x file size (4mb to 8mb) but I don't like how Kindlegen handles formatting and ToC.

I've accepted the fact that MOBI files are typically 2x as big as EPUBs but I'm confused about why Calibre is kicking it up 4x. Does anyone have any advice on this? At this point I would be happy with a 2x file size from Calibre.

In my experience, text is more compact in MOBI than it usually is in ePub resulting in smaller files if, and only if, the ebook has only a minimal cover and includes a lot of text (novel +).

On the other hand, images don't seem to compress/compact as well in MOBI, at least when calibre does the ePub to MOBI conversion. The difference is largest when the images in the ePub are GIFs. When I build illustrated ebooks with a number of images to ePubs are usually smaller.

I recently built both ePub and MOBI version of The Rome Express by Griffiths that include a decent sized cover, one frontis JPEG image and two small images (GIF) on the title page along with an ornament (GIF) at the beginning of each chapter. The ePub is 248k and the MOBI is only slightly larger at 313k.

A similar pair that include a number of nearly full screen JPEG images, Three Men in a Boat by Jerome, have a much greater difference. The ePub is 1508k while the MOBI is over twice as large at 3462k.

A third example is Lysistrata by Aristophanes. Here, all of the illustrations, except the cover, are GIFs in the ePub. The MOBI version is nearly 3x the size of the ePub, 632k vs, 1853k.

You can download these books from my Lakemere Press site if you want to examine them.

A third example is Lysistrata by Aristophanes. Here, all of the illustrations, except the cover, are GIFs in the ePub. The MOBI version is nearly 3x the size of the ePub, 632k vs, 1853k.

What appears to be happening is that it may not even matter what file type or size you put in. I think the conversion is taking the actual image pixel data and saving it as a jpeg with the highest quality compression. Here's why I think this:

I took my original EPUB with the larger gif file sizes--roughly 9MB. I ran that through Calibre and it came out to the same 15MB MOBI as when I converted the 4MB EPUB. I then did MOBI to MOBI on that file and it dropped down to 11MB just like the 4MB EPUB did.

It's just crazy that a 9MB and 4MB version of the same EPUB would both come out to be the same 15MB size MOBI.

Just for fun I did an experiment with Lysistrata. I took the same source ePub that I had fed to calibre for conversion to MOBI and fed it to Kindle Previewer 2 (aka Fire Previewer) so that its KindleGen2 engine would create a KF8 MOBI. These result was smaller than calibre's:

calibre converts all images in MOBI to JPEG. And no, it's not a bug, various older MOBI renderers have problems with GIF and PNG. I always go for compatibility over size. That is not going to change.

Hi Kovid,

I totally understand the compatibility thing. Do you have any recommendations for decreasing the file size of the Calibre image conversions? I've got 10k gifs but even when I save them as jpeg with 0 quality settings they are coming up at 30k+.

It's brutal because I'm looking at having to pay a $2.25 "delivery fee" for the 15MB MOBI of a $4.99 book.

Use the free wifi delivery instead of the 3G one. Use @free.kindle.com as the email address. If you mean you want to upload a MOBI created with calibre, then there isn't anyway to do it, short of running calibre from source and changing the code.